1. Technical Field
This invention relates to pattern delineation and, in particular, to pattern delineation involving electric discharges.
2. Art Background
Many different applications require that fine lines be delineated ("traced") on substrate materials. Three typical examples of such applications are VLSI reticle mask fabrication, hybrid circuit fabrication and construction of diffraction gratings. Although the need for fine line tracing has spawned several successful technologies, new and improved tracing technologies are still sought in the art. Two of the more successful tracing technologies are lithography and mechanical scribing.
Exemplary of articles produced by lithography are reticle masks. A metal (usually chromium) plated glass substrate is coated with a radiation-sensitive resist. The resist is then exposed using a well-collimated electron beam or laser. This exposure process serves to trace predetermined patterns in the resist, inducing localized chemical reactions in the exposed pattern. The resist is then developed in a suitable solvent leaving regions of bare metal corresponding to the laser (or electron beam) pattern. The remaining metal is covered with resist. The unprotected, bare metal is subsequently etched through to the transparent substrate leaving behind a pattern of resist-coated metal corresponding to the laser or electron beam pattern. The resist is then removed and a reticle mask composed of transparent substrate regions and opaque metallized regions remains. The reticle mask is then employed as an optical master in subsequent lithographic steps for integrated circuit fabrication.
The conventional lithographic process for fabrication of reticle masks yields excellent fine linewidths (well under 1 micron with electron beam technology; 3 microns with laser technology), but has a very high capital cost due to the complicated electron beam or laser apparatus required to trace the pattern. A resistless direct thermochemical laser etching process that may also be suitable for tracing has been developed (see U.S. Pat. No. 4,283,259, issued Aug. 11, 1981). However, this laser technique also suffers from the high capital requirements of the conventional lithographic systems.
The formation of fine lines in hybrid and printed circuits is also generally accomplished by conventional lithographic techniques. However, lithography is hard-pressed to cope with some of the extremely nonplanar features frequently found in hybrid circuits. The optical system that projects the mask image on the resist cannot keep the entire nonplanar hybrid structure in its focal plane. Furthermore, the resist at the nonplanar steps is subject to self-masking from the vertical mask image rays. Thus, for some applications, not only high capital requirements but also technical limitations are associated with lithography.
In contrast to the relatively expensive fine-line lithographic technology developed for integrated circuit fabrication, mechanical scribing technology has far lower capital and operating costs. Unfortunately, it is generally far less capable of scribing fine lines. One typical apparatus yields 20 micron resolution specifically for evaporated gold surfaces. (See, for example, Albrecht-Buehler, J. Cell Bio., 80:53 (1979).) Highly sophisticated scribing technologies have been embodied in dividing engines. Mechanical dividing engines have scribed gratings of up to 100,000 lines/inch resolution (0.25 micron linewidth). See Hardy and Perrin, The Principles of Optics, p. 563, McGraw-Hill (1932). However, these dividing engines are expensive and inflexible, designed only to trace parallel grooves generally only in soft substrate materials.
Mechanical scribing, like most machining techniques, uses one piece of solid material to shape another. Materials can also be shaped by "electromachining" processes such as electric discharge machining (EDM), also known as electrospark or electro-erosion machining. This technique, which potentially works on all electric conductors, relies on pulse plasma vaporization of the material being worked. The workpiece is grounded and an arc is struck between an electrode and the piece. If the piece is a sufficiently good conductor, the arc forms a plasma which vaporizes material in the close proximity of the arc. If conditions are chosen so that the plasma is extinguished shortly after the arc is struck, reasonable control over the removal process can be obtained. This technique is useful, for example, to bore, grind, and slot metal. Another electromachining process is called electrochemical machining (ECM). This process is similar to EDM except it uses electro-etching rather than plasma discharge to remove material from a workpiece.
Both ECM and EDM have significant limitations. The conventional wisdom in EDM holds that the working electrode must have the inverse shape and correct dimensions of the form to be generated during machining. The simplest and strongest branding iron is a cylinder, and indeed 10 micron holes have been pierced in material using the EDM process with fine-drawn wire electrodes (Japanese Patent Application 982-33922, filed Feb. 24, 1982 (Sato et al)). However, a more complicated micro-branding iron would be very difficult to fabricate and probably would not have enough mechanical integrity to successfully delineate surfaces. As mentioned, translation of a 10 micron cylindrical wire in the fashion of a mill to produce a delineated pattern rather than a hole presents extraordinary mechanical integrity problems.
In summary, the need for drawing fine lines has only been satisfied by expensive lithographic or laser technologies. Mechanical scribing does not yield the 10 micron lines required for reticle mask technology in a flexible or inexpensive manner. With the exception of EDM microdrilling, existing electromachining techniques do not appear appropriate for any small-scale work and the sole ability to drill holes is of limited use. Although inexpensive alternatives to lithography have been sought, they have not been found.